1. Field of the Invention
This invention relates to data recording codes having predetermined characteristics, particularly to run-length-limited (d,k) codes for optical recordings of data and minimum interference with associated servo systems.
2. Description of Related Art
Recording binary information in its original form presents problems because of the nature of the recording channel which is analogous to a communication channel and includes the circuits for recording and retrieving stored data. A channel can transmit and receive (record and retrieve) data at a rate termed the channel capacity which depends on bandwidth, noise, and other channel parameters.
The recording channel, hereinafter referred to as channel, has the same kinds of characteristics as communication channels. Magnetic recorders, for example, have a low frequency limit because of the inability to record and to retrieve d-c signals using magnetic media. Optical recorders, on the other hand, have the capability of recording and reading d-c signals, but a-c coupled amplifiers are usually used to amplify the signals because they provide more gain with less complicated circuitry than d-c coupled amplifiers. Because of a-c coupling in the amplifiers, however, optical readers also have a low frequency limitation.
The timing signals in most data recordings are derived from the same channel as the data and are extracted by filters, special logic circuits, or the like. In optical recordings, the servo signals, used both to keep the lens system at best focus, and to position the head properly over the desired data track, is a low frequency signal. Data containing low frequency spectral power interfere with the servo signal.
It is well known to record data having a spectral density that matches the transfer characteristics of the channel. This may sometimes cause a higher than average error rate for data sequences having certain characteristics but it enables a channel to operate at higher capacity. Some coding schemes for data recording are described in the following references:
1. K. A. Immink, "Modulation Systems for Digital Audio Disks with Optical Readout," Proc.IEEE Int. Conf. on Acoustics, Speech and Signal Processing (Atlanta, 1981), pp. 587-589. PA0 2. A. M. Patel, "Zero Modulation Encoding for Magnetic Recording," IBM J. Res. and Dev., Vol. 19, No. 4, July, 1975, pp. 366-378. PA0 3. J. C. Mallinson and J. W. Miller, "Optimal Codes for Digital Magnetic Recording," The Radio and Electronic Engineer, Vol. 47, No. 4, April, 1977, pp.172-176. PA0 4. G. L. Pierobon, "Codes for Zero Spectral Density at Zero Frequency," IEEE Trans. Info. Th., Vol. IT-30, No. 2, March, 1984, pp.435-439. PA0 5. R. Adler, D. Coppersmith, and M. Hassner, "Algorithms for Sliding Block Codes," IEEE Trans. Info. Th., Vol. IT-29, No. 1, January, 1983, pp. 5-22.
The references describe recording techniques such as NRZ (Non-Return to Zero) code, recording bits by a transition, i.e., change of voltage level, on the media for each recorded bit that differs from its predecessor. In Reference (3), it is described as NRZ(LEVEL) or NRZ(L). NRZ(MARK) or NRZ(M), on the other hand, is a recording technique by which binary ones are recorded as transitions in the middle of a data interval (bit cell) and binary zeros are ignored. (A bit cell or data interval is the distance or time interval on the channel or media assigned to each bit. The bit cell size is limited by the channel bandwidth.)
Also described in the references are encoding techniques which substitute one binary word for another so that the undesirable spectral qualities of recorded data bit sequences can be eliminated by substituting encoded bit sequences having spectral characteristics that more closely match the transfer characteristics of the channel.
One desirable characteristic is a zero d-c spectral component, i.e., the Fourier transform of the signal over some large interval has no constant value. Physically, if binary ones are represented by +v and binary zeros, by -v, the area above the baseline and the area below will be equal over given intervals, corresponding to the absence of a d-c spectral component represented by a Fourier transform constant value of zero. Alternating ones and zeros, for example, have no d-c spectral component over intervals which are multiples of two bit cells.
Since arbitrary input data can have long sequences of binary ones or binary zeros, theoretically infinitely long for an infinite number of data bits, there will be a d-c spectral component if the encoded bits are equal to the data bits. The encoded bits are selected so that the number of binary ones or binary zeros that can occur in sequence is limited. In an y-bit encoded word, the minimum number of zeros that are allowed to occur in sequence is designated by d and the maximum, by k. Such codes are called Run-Length-Limited (RLL) codes and are denoted by (d,k). A (2,7) code means that encoded binary ones must be separated by at least two and not more than seven binary zeros.
The above references describe such codes, also known as group or block codes, which can be selected according to (d,k) criteria so that the d-c spectral component averages to zero.
The described prior art techniques suitable for use with magnetic media are not satisfactorily applicable to optical recordings using pit-per-transition techniques because of differing channel characteristics. The invention described and claimed herein is directed to a method that is especially adapted to encoding for the PPT optical recording art.